Nb3 Al Group superconductor containing ultrafine Nb2 Al particles

ABSTRACT

Object of the present invention is to obtain a Nb 3  Al group superconductor having a high critical current density under a whole range of magnetic field from low to high such as 20 T level, manufacturing methods thereof, a Nb 3  Al group superconducting precursory composition, and a magnet for high magnetic field. In a process for manufacturing Nb 3  Al phase by a diffusion reaction of Nb 2  Al phase and Nb phase, a part of the Nb 2  Al phase is remained and dispersed in the Nb 3  Al phase homogeneously as for magnetic flux pinning centers for a high magnetic field. As for a method for dispersing the Nb 2  Al phase homogeneously, a Nb 3  Al group superconducting precursory composition obtained by dispersing Nb particles and Nb 2  Al ultrafine particles by a mechanical alloying method is used, and further, by a conventional method for generating Nb 3  Al phase by a diffusion reaction of Nb and an aluminum alloy, the object of the present invention can be achieved. In accordance with the present invention, a high magnetic field such as 20 T level which has never been achieved with only superconductor at 4.2K can be generated economically with a more compact apparatus than ever, and a coil for generating a high magnetic field can be manufactured with only the Nb 3  Al group superconductor. Therefore, the present invention is significantly effective for a nuclear fusion apparatus of a magnetic confinement type.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to Nb₃ Al group superconductors,particularly, to Nb₃ Al group superconductors and manufacturing methodthereof, Nb₃ Al group superconductive precursory compositions, andsuperconducting magnets for generating high magnetic field preferablefor nuclear fusion apparatus, nuclear magnetic resonance apparatus, andhigh magnetic field generating apparatus all of which require highmagnetic field.

2. Description of the Prior Art

Nb₃ Al group superconductors have been manufactured, as described inApplied Physics Letters, vol. 52 No. 20 p 1724-1725 (1988. 5.16), in amanner that a plurality of aluminum alloy core materials and niobiummatrix material are made to an ultrafine multicore cable by a compositemanufacturing method, and subsequently, the cable is treated withdiffusion heat treatment at 750°-950° C.

Another manufacturing method in which Nb₂ Al powder and Nb powder aremixed, fabricated, and sintered at 1,300°-1,400° C. to be Nb₃ Al isdisclosed in The Proceedings of 45th on Cryogenics and Superconductivity(Chiba, Japan, May 14-16, 1991), p 246.

Further, other manufacturing method is disclosed in JP-A-3-283322(1991), in which a composite body is formed by filling mixed powder ofNb₂ Al alloy and niobium into a metallic tube made from niobium or Nbbase alloy, a core material is formed by cold working of the compositebody, and subsequently, Nb₃ Al superconductor is obtained by heattreatment of the core material for generating Nb₃ Al which changes thecold worked powder to Nb₃ Al.

Among the above described prior art, the composite manufacturing methodhad a problem that the Nb₃ Al ultrafine multicore cable obtained bynormal diffusion heat treatment had low critical temperature and lowcritical magnetic field, and the critical current density decreasedremarkably under a high magnetic field condition such as 20 T level. Themethod by fabrication and sintering of Nb₂ Al powder and Nb powdernecessitated sintering at high temperature, and accordingly, it had aproblem although it enhanced yielding of Nb₃ Al that crystal grain sizeof Nb₃ Al increased and the critical current density decreased under lowand medium magnetic field conditions.

That means, a case of Nb₃ Al group superconductors, high criticaltemperature and high critical magnetic field depending oncharacteristics of Nb₃ Al material itself can not be realized, andaccordingly, the critical current density under a high magnetic fieldcondition is low, and the Nb₃ Al superconductors do not become cablematerials for practical use. Generally speaking, it is well known thatthe critical current density of Nb₃ Al group superconductor depends onsize of the crystal grain, although a Nb₃ Al superconducting phase bythe above described conventional composite manufacturing method includesfine crystal grains and many magnetic flux pinning centers which areeffective under a relatively low magnetic field condition, the Nb₃ Algroup superconductors have low critical current density under a highmagnetic field condition because of shifting the composition ratio ofniobium and aluminum etc.

Further, it is well known that Nb₂ Al non-superconductive phase acts asan effective magnetic flux pinning center under a high magnetic field.However, the conventional diffusion reaction of niobium and aluminumgenerates aluminum enriched NbAl₃ phase first in accordance with thermalequilibrium, and subsequently generates Nb₂ Al phase and superconductiveNb₃ Al phase through Nb₂ Al phase. Therefore, a great effort is devotedto form Nb₃ Al single phase, but Nb₂ Al non-superconductive phase doesnot exist in the single phase.

Further, various methods for generating only superconductive Nb₃ Alphase having a large amount of grain boundaries which are magnetic fluxpinning centers in Nb₃ Al phase, that means fining the grain size, havebeen attempted, but any method has not succeeded. Moreover, a method inwhich a diffusion reaction is performed at high temperature with Nb₂ Alphase and niobium as starting materials was proposed. But, although theproposed method relatively facilitates formation of Nb₃ Al phase, grainsize remains still large. The method disclosed in JP-A-3-283322 (1991)neither indicate any improvement on critical current density under ahigh magnetic field condition, nor any special consideration on arelationship between the Nb₂ Al non-superconductive phase and themagnetic flux pinning centers under a high magnetic field condition.

SUMMARY OF THE INVENTION 1. Objects of the Invention

One of the objects of the present invention is to provide Nb₃ Al groupsuperconductors having high critical current density under high magneticfield condition such as 20 T level and manufacturing method thereof, Nb₃Al group superconductive precursory compositions, and superconductingmagnets for generating high magnetic field.

Another object of the present invention is to provide Nb₃ Al groupsuperconductors having high critical current density under a wholecondition from low magnetic field to high magnetic field andmanufacturing method thereof, Nb₃ Al group superconductive precursorycompositions, and superconducting magnets for generating high magneticfield.

2. Methods of Solving the Problems

In order to achieve the above objects, the present invention provides acomposition wherein Nb₂ Al phase is dispersed in Nb₃ Al phase in the Nb₃Al group superconductors generated by a diffusion reaction of Nb₂ Al andNb or Nb alloys. Preferable structure for improving the critical currentdensity under a high magnetic field condition in the above method issuch that a grain size of the Nb₂ Al phase is utmost 0.1 μm in averagediameter and an interval between each grain is in a range of 0.01-0.1μm.

In order to obtain the Nb₃ Al group superconductors having the abovedescribed composition and structure, a method wherein Nb particles andNb₂ Al fine particles are dispersed and mixed each other by a mechanicalalloying method is preferable because the method makes it possible toobtain a large amount of the Nb₃ Al group superconductive precursorycompositions having a most homogeneously dispersed structure. Thatmeans, dispersing Nb₂ Al fine particles having utmost 0.1 μm in averagediameter in Nb particles having 1-100 μm in average diameter by themechanical alloying method enables the hard Nb₂ Al fine particles bedispersed in relatively soft Nb particles homogeneously.

Further, in order to obtain a superconductor having a large currentcarrying capacity for practical use, it is preferable to perform adeformation processing on the above Nb₃ Al group superconductiveprecursory composition with deformation ratio at least 1000, andsubsequently, to treat by a diffusion reaction at 1,000°-1,800° C.

Besides, the Nb₃ Al group superconductors having an electro-magneticallystable multifilamentary structure can be obtained by deformationprocessing of the Nb₃ Al group superconductive precursory composition totubular or linear shapes, subsequent deformation processing forcomposing with Al alloy matrix, and a diffusion reaction.

In the present invention, the Nb₃ Al group superconductors can have highcritical current densities in a whole region from a low magnetic fieldto a high magnetic field by artificially controlling and usingsubstantially two kinds of different magnetic flux pinning centers.

In accordance with the present invention, a part of Nb₂ Al phase andniobium phase remain without reacting each other although the Nb₂ Alphase and the niobium phase are used as starting materials, and theremained Nb₂ Al phase has a function as a magnetic flux pinning centerunder a high magnetic field. That means, in a chemical reactionexpressed by the following equation, all of reactants are notnecessarily converted to Nb₃ Al phase but a part of which remain as Nb₂Al phase which disperses in Nb₃ Al phase of the Nb₃ Al groupsuperconductor by terminating the reaction before completing thereaction;

    2 Nb.sub.2 Al+2 Nb→Nb.sub.3 Al+Nb.sub.2 Al+Nb→2 Nb.sub.3 Al

Accordingly, the objects of the present invention can be achieved by thediffusion reaction of lower temperature and shorter time than that ofthe reaction for converting all of the reactants to Nb₃ Al phase.

In the present invention, it was revealed experimentally that thestructure wherein a grain size of the Nb₂ Al phase dispersed in the Nb₃Al phase has utmost 0.1 μm in diameter and an interval between eachgrain is in a range of 0.01-0.1 μm is significantly preferable state formaking the Nb₂ Al phase have a function as magnetic flux pinning centerunder a high magnetic field condition. Because of remarkable difficultyin dispersing Nb₂ Al phase in Nb₃ Al phase homogeneously by conventionalpowder metallurgy, a mechanical alloying method wherein Nb₂ Al fineparticles having utmost 0.1 μm in diameter are dispersed in Nb particleshaving 1-100 μm in diameter was adopted in the present invention. As aresult, it became possible to disperse the particles homogeneously in aform that a plurality of the Nb₂ Al fine particles 2 bite into the Nbparticle 1 as shown in FIG. 2 because of softness of the Nb particle.Further, it was confirmed that the Nb particle was crushed by thesubsequent fabrication.

By performing a deformation processing on the Nb₃ Al groupsuperconductive precursory composition obtained by the above describedmanner with deformation ratio at least 1000, and subsequent treating bya diffusion reaction at 1,000°-1,800° C., the Nb₃ Al groupsuperconductor was obtained wherein a part of respective Nb₂ Al phase 2and Nb phase 1 reacts to generate Nb₃ Al 3 phase, and the Nb₂ Al phase 2is homogeneously dispersed in the Nb₃ Al phase 3 as schematically shownin FIG. 1.

As explained above, the present invention generates Nb₃ Al phase atlower temperature than that of the conventional method. Accordingly, theNb₃ Al phase acts as magnetic flux pinning centers under a low magneticfield condition, and the Nb₂ Al phase dispersing homogeneously in theNb₃ Al phase acts effectively as magnetic flux pinning centers under ahigh magnetic field condition. As for the Nb phase, Nb alloys can beused in a range that the above reaction is not disturbed.

Further, a conventional reaction of niobium or Nb alloys with aluminumor Al alloys to generate Nb₃ Al as expressed by the following equationcan be superimposed to the above reaction effectively in practical use;

    2 Nb.sub.2 Al+5 Nb+Al→2 Nb.sub.3 Al+Nb.sub.2 Al+Nb

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross section of the Nb₃ Al group superconductorfor explaining a structure of the present invention,

FIG. 2 is a schematic cross section of the Nb₃ Al group superconductiveprecursory composition for explaining a structure of the presentinvention,

FIG. 3 is a graph showing a magnetic field/critical current densitycharacteristics at 4.2K of the Nb₃ Al group superconductor manufacturedby the prior art and the present invention, and

FIG. 4 is a schematic cross section of a superconducting coil forexplaining a structure of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The embodiments of the present invention are explained hereinafterreferring to figures in comparison with the prior art. But the presentinvention is not restricted by the embodiments.

Embodiment 1

Niobium powder having about 50 μm in diameter and Nb₂ Al fine powderwhich was obtained by arc-melting and pulverizing to 0.07 μm in averagediameter were mixed by approximately 1:1, and the mixed powder wasconverted to integrated powder by a mechanical alloying method using ahigh energy ball milling in an inert gas atmosphere. The integratedpowder had a cross section shown in FIG. 2 in which a plurality of hardNb₂ Al fine powder 2 were dispersed homogeneously in the soft and largeNb particle 1, and the Nb particle was deformed to a flat shape by alarge plastic deformation.

Next, the integrated powder, that is the Nb₃ Al group superconductiveprecursory composition, was put into a tube made from niobium having11.0 mm in outer diameter and 7.0 mm in inner diameter and sealedhermetically. Then, the tube was drawn to be wire having 1.0 mm in outerdiameter for obtaining composite multicore cables. Deformation ratio(decreasing fraction of cross sectional area) at the time was about5×10⁴. Subsequently, the wire was heated at 1,200° C. for 30 minutes fordiffusion reaction to be Nb₃ Al group superconductor, and then, copperwas plated on surface of the wire for about 10 μm thick.

Subsequently, samples were cut out from the superconductor, criticaltemperatures were measured under various temperatures by a resistivemethod, and critical currents were measured under various magneticfields at 4.2K.

As for the critical temperature, 17.9K at middle point temperature wasobtained, and it revealed that the superconductor manufactured by thepresent invention could be improved by 2.3K in comparison with 15.6K ofthe Nb₃ Al conductor manufactured by the conventional compositemanufacturing method. The critical current was taken as the current at apoint when the voltage 1 μV/cm was generated, and the critical currentdensity was calculated by per cross sectional area without copperstabilizing material. The result is shown in FIG. 3. The curve line 4 inFIG. 3 indicates magnetic field of the present embodiment/criticalcurrent density characteristics, and the curve line 5 indicates magneticfield of Nb₃ Al ultrafine multicore material by the conventionalcomposite manufacturing method/critical current density characteristics.As the figure reveals, the superconductor obtained by the method of thepresent invention has higher critical current densities than thatobtained by the conventional method, especially at high magnetic fieldin a range higher than 15-16 T. For instance, in a magnetic field of 20T, the superconductor of the present invention is the Nb₃ Alsuperconductor for high magnetic field having almost 1000 times criticalcurrent density in comparison with that of the prior art.

Embodiment 2

Nb₃ Al group superconducting precursory composition prepared bydispersing and mixing of Nb particles and Nb₂ Al fine particles by themechanical alloying method as same as the embodiment 1 was used formanufacturing composite single core wire by fabricating a hollowedcylinder having 11.0 mm O.D. and 7.0 mm I.D., inserting an alloy rod ofAl-Mg 5 atomic per cent having 6.5 mm in diameter into the abovehollowed cylinder, and drawing to be a wire having 3.0 mm O.D.Subsequently, a composite multicore wire was manufactured by bundling 37of the above composite single core wires, inserting the bundle into ahollowed niobium cylinder having 25.5 mm O.D. and 21.5 mm I.D., anddrawing to be a wire having 1.0 mm O.D. A deformation ratio of thedrawing was about 8×10³. And, a diameter of the Al-Mg 5 atomic per centcore material (matrix) was about 70 μm. Subsequently, after the abovewire was treated at 1,200° C. for 30 minutes for a diffusion reaction tobe the Nb₃ Al group superconductor, copper was plated on surface of thewire for about 10 μm thick, and further a heat treatment at 850° C. for50 hours in vacuum was performed.

As same as the embodiment 1, critical temperatures and magneticfield/critical current density characteristics of samples were measured.The result of the measurement revealed that the critical temperature was17.5K, somewhat lower than that of the embodiment 1, and, as for themagnetic field/critical current density characteristics, large criticalcurrent densities were obtained in a range of magnetic field less than18 T as shown by the curve line 6 in FIG. 3. This is because of yieldingNb₃ Al phase which is generated by a reaction of Nb phase and Al-Mgalloy matrix phase in addition of Nb₃ Al phase which is generated by areaction of Nb phase and Nb₂ Al phase.

Consequently, in comparison with the superconductor of the prior artshown by the curve line 5 in FIG. 3, the superconductor of the presentinvention was improved remarkably on the critical current density in awhole range of the magnetic field.

Embodiment 3

Nb₃ Al group superconducting precursory composition prepared bydispersing and mixing of Nb particles and Nb₂ Al fine particles by themechanical alloying method as same as the embodiment 1 was used formanufacturing composite single core wire by fabricating a rod having 6.5mm O.D., inserting the above rod into a hollowed cylinder of Al-Mg 5atomic per cent alloy having 11.0 mm O.D. and 7.0 mm I.D., claddingouter surface of the hollowed cylinder with a niobium tube, and drawingto be a wire having 3.0 mm O.D. Subsequently, a composite multicore wirewas manufactured by bundling 37 of the above composite single corewires, inserting the bundle into a hollowed niobium cylinder having 25.5mm O.D. and 21.5 mm I.D., and drawing to be a wire having 1.0 mm O.D.Subsequently, after the above wire was treated at 1,200° C. for 30minutes for a diffusion reaction to be the Nb₃ Al group superconductor,copper was plated on surface of the wire for about 10 μm thick, andfurther a heat treatment at 850° C. for 50 hours in vacuum wasperformed.

The result of measurement of critical temperatures and critical currentdensities on the samples revealed similar characteristics with those ofthe embodiment 2, and, in comparison with the superconductor of theprior art, the superconductor of the present invention was improvedremarkably on the critical current density in a whole range of themagnetic field.

Embodiment 4

Nb₃ Al group superconducting precursory composition prepared bydispersing and mixing of Nb particles and Nb₂ Al fine particles by themechanical alloying method as same as the embodiment 1 was used formanufacturing a composite wire by fabricating a plate, rolling the plateto be a foil of 0.1 mm thick, rolling an Al-Mg 5 atomic per cent alloyto be a foil of 0.1 mm thick, laminating the above two kinds of foils,winding up the laminate to be a roll so that the cross section forms aspiral, inserting the roll into a niobium tube having 25.5 mm O.D. and21.5 mm I.D. and drawing the above tube to be the composite wire having1.0 mm in diameter.

Subsequently, after the above wire was treated at 1,200° C. for 30minutes for a diffusion reaction to be the Nb₃ Al group superconductor,copper was plated on surface of the wire for about 10 μm thick, andfurther a heat treatment at 850° C. for 50 hours in vacuum wasperformed.

The result of measurement of critical temperatures and critical currentdensities on the samples revealed similar characteristics with those ofthe embodiment 2, and, in comparison with the superconductor of theprior art, the superconductor of the present invention was improvedremarkably on the critical current density in a whole range of themagnetic field.

Embodiment 5

A Nb₃ Al superconductor of 0.25 mm thick and 5.0 mm wide wasmanufactured in accordance with the method explained in the embodiment2, and was wound tightly in the region (I) of the hollowed cylindricalsuperconductor such as shown in FIG. 4. A conventional (Nb, Ti)₃ Snultrafine multicore superconductor was wound tightly in the region (II)as well. The above superconducting coil was a bobbin, in which the coilin the region (I) was 50 mm I.D., 130 mm O.D., and 400 mm in axiallength, and the coil in the region (II) was 130 mm I.D., 500 mm O.D.,and 400 mm in axial length. The region (I) and (II) were electricallyconnected in a series, and were energized in liquid helium by a powersource.

As a result, a magnetic field of 19.5 T could be generated at the centerregion of the coil. If the region (I) was also wound by the conventional(Nb, Ti)₃ Sn ultrafine multicore superconductor, the maximum magneticfield at 4.2K was about 17 T, and accordingly, it was revealed that thepresent invention could increase remarkably the magnetic field generatedby the superconductor with a coil having a same cross section area.Further, it can be said a significant technical advantage that such ahigh magnetic field as ever been achieved at 4.2K can be achieved by thepresent invention.

As above explained, the present invention makes it possible to generatea high magnetic field such as 20 T level at 4.2K, which has never beenachieved with a superconducting magnet, with a more compact apparatusthan ever economically. Further, as the present invention uses Nb₃ Algroup superconductor having high current density in a whole magneticfield from low to high magnetic fields, winding can be performed withonly single kind of superconductor, and accordingly, the winding isrelatively facilitated. Moreover, becoming it possible to generate ahigh magnetic field with only the Nb₃ Al group superconductor can solveinstantly a problem of induced radioactivity in a nuclear fusionapparatus of a magnetic confinement type, and its advantage isremarkable.

What is claimed is:
 1. A superconductor generated by a diffusion reaction between Nb₂ Al and Nb, said superconductor comprising a Nb₂ Al phase dispersed in a Nb₃ Al phase, said Nb₂ Al phase having an average diameter of utmost 0.1 μm and the interval between each grain of said Nb₂ Al phase being in a range of 0.01-0.1 μm, said superconductor being produced by a method comprising the steps of:mixing niobium powder and Nb₂ Al powder in a weight ratio of about 1:1 to form a precursor composition for the superconductor, deforming said precursor composition with a deformation ratio of at least 1,000 and subsequently performing a diffusion reaction with the deformed precursor composition at 1,000°-1,800° C.
 2. A superconductor wherein magnetic flux pinning centers of Nb₂ Al which have a different phase from a superconducting phase of Nb₃ Al and are effective in a high magnetic field are dispersed in the superconducting phase of Nb₃ Al having effective magnetic flux pinning centers in a low magnetic field, said different phase of Nb₂ Al having an average diameter of utmost 0.1 μm and the interval between each grain of said different phase of Nb₂ Al being in a range of 0.01-0.1 μm; said superconductor being obtained by deformation of a precursor composition at a deformation ratio of at least 1,000, said precursor composition containing particles of Nb₂ Al and particles of Nb in a weight ratio of 1:1, and by effecting a diffusion reaction of the deformed precursor composition at 1,000°-1,800° C. 